Dense mode coding scheme

ABSTRACT

A lookup table for searching for a longest prefix match for a key is disclosed. The lookup table provides a match for a key in a single search cycle. The number of matches stored in the lookup table is maximized by storing each match in only one location in the lookup table. The binary tree is divided into a plurality of levels and each level has a plurality of subtrees. A subtree descriptor stored for a subtree includes a field for each node in the subtree. The state of the field indicates whether an entry for the node is stored in the table. The bit vector allows indexing of the single match stored for the key.

BACKGROUND OF THE INVENTION

[0001] The Internet is a set of networks connected by routers. A router maintains a routing table that indicates for each possible destination network, the next hop to which a received data packet should be forwarded. The next hop may be another router or the final destination.

[0002] An Internet Protocol (“IP”) data packet received at a port in a router includes an IP destination address. The IP destination address is the final destination of the IP data packet. Currently there are two versions of IP, IP version 4 (“IPv4”) and IP version 6 (“IPv6”). IPv4 provides a 32-bit field in an IP header included in the data packeted for storing the IP destination address. The router forwards a received data packet connected to a next-loop router, or the final destination if the destination is the local network, dependent on the IP destination address stored in the IP header.

[0003] A 32-bit IPv4 destination address provides 4 billion possible routes. An Internet router typically stores 50,000 of the 4 billion possible routes. However, the number of stored routes will increase with the growth of the Internet and the widespread use of IPv6.

[0004] Originally, the IP address space was divided into three classes of IP addresses; A, B and C. Each IP address space was divided into a network address and a host address. Class A allowed for 126 networks and 16 million hosts per network. Class B allowed for 16382 networks with 64,000 hosts per network and class C allowed for 2 million networks with 256 hosts per network. However, dividing the IP address space into different classes reduced the number of available IP addresses. Class C only allowed a maximum of 256 hosts per network which is too small for most organizations. Therefore, most organizations were assigned a Class B address, taking up 64,000 host addresses which could not be used by other organizations even if they were not used by the organization to which they were assigned. Hosts in an organization with a Class B IP address all store the same network address in the 16 Most Significant Bits (“MBSs”), for example, 128.32.xx.xx.

[0005] Classless InterDomain Routing (“CIDR”) was introduced to free up unused IP host addresses. The remaining unused networks are allocated to organization in variable sized blocks. An organization requiring 500 addresses gets 500 continuous addresses. For example, an organization can be assigned 500 available addresses starting at 128.32.xx. The number of routes stored by a router has increased since the introduction of Classless InterDomain Routing. Classless InterDomain Routing requires longest prefix matching to find the corresponding route instead of searching for a matching network address in order to find the corresponding next hop for the IP destination address. For example, a search can no longer stop after the 16 MSBs of a Class B IP address, for example, 128.xx.xx because 128.32.4.xx may be assigned to another organization requiring a different next hop.

[0006] One method for searching for a longest prefix match for a key is through the use of a binary tree search. A binary tree search matches a 32-bit input bit by bit down to 32 levels, requiring 32 searches to find the entry matching the 32-bit key. Another method for searching for a match is through the use of a Patricia tree. A Patricia tree reduces the number of searches required if there are no entries down a leaf of the binary tree.

[0007] Yet another method for efficiently searching for a next hop associated with an IP destination address is described in PCT application Serial Number PCT/SE98/00854 entitled “Method and System for Fast Routing Lookups” by Brodnick et al. filed on May 11, 1998. The method described by Brodnick reduces the number of next hops stored by not storing duplicate routes. By reducing the number of next hops, the memory requirement is reduced so that a route lookup table can be stored in fast cache memory.

[0008] Brodnick et al. divides the binary tree into 3-levels. Dividing the binary tree into 3-levels reduces the number of searches to three. The indexed entry in the first level indicates whether the search can end at the first level with the route taken from the entry, or the search must continue to a subsequent level using a further portion of the IP destination address.

[0009]FIG. 1A illustrates a prior art 64K (65536) bit map representing the first level of a binary tree. A 64K bit map 30 represents the leaves or nodes 44 of the binary tree at depth 16, with one bit per node 44. The bit map is divided into bit-masks of length16. There are 2¹²=4096 bit masks in the 64k bit map. One bit mask is shown in FIG. 1A. A bit in the bit map 30 is set to ‘1’ if there is a subtree or a route index stored in an array of pointers corresponding to the node 44. A bit in the bit map 30 is set to ‘0’ if the node shares a route entry with a previous node 44.

[0010] A bit is set in the bit map 30 if the node does not share the route entry the previous node 44. Bit 4 is set ‘1’ indicating that there is a subtree or route index corresponding to the node 44. Bit 5 is set ‘0’ indicating that the node corresponding to bit 5 shares the subtree or route index with the node corresponding to bit 4. Bit 6 is set ‘1’ indicating that the node corresponding to bit 6 does not share a subtree or route index with bits 4 and 5. Bit 6 corresponds to a node sharing the default route. Bits 6 and 12 set to ‘1’ correspond to the default route.

[0011]FIG. 1B illustrates a prior art lookup table implemented in cache memory. The lookup table includes an array of code words 36, an array of base indices 34 and a map table 40. A 32-bit IP address 38 is also shown in FIG. 1B. A codeword 46 is stored in the array of code words 36 for each bit mask in the bit map 30 (FIG. 1A). The code word 46 includes a six-bit value 46 a and a 10-bit offset 46 b. A base index 42 is stored in the array of base indices 34 for every four code words 46 in the array of code words 36.

[0012] The array of code words 36, array of base indices 34 and map table 40 are used to select a pointer in an array of pointers (not shown). The pointer stores a route index or an index to perform a further search.

[0013] A group of pointers in the array of pointers is selected by selecting a code word 46 in the array of code words 36 and a base index 42 in the array of base indices 34. The code word 46 is selected using the first 12 bits 50 of the IP address 38. The base index 42 is selected using the first 10 bits 48 of the IP address 38. The correct pointer in the group of pointers is selected using the map table 32.

[0014] The 10-bit value 46 b in the selected code word 36 is an index into the map table 32. The map table 32 maps bit numbers within a bit-mask to 4-bit offsets. The offset specifies the pointer within the selected group of pointers in the array of pointers. The 10-bit value 46 b selects the row in the map table 32 and bits 19:16 of the IP address 52 selects the 4-bit offset 54.

[0015] An offset to specify a pointer is stored in the map table 32 for each ‘1’ stored in the bit mask 30 (FIG. 1A). Thus, two separate location offsets to specify a default pointer corresponding to bits 6 and 12 in the bit mask 30 are stored in two separate locations in the map table 32. Storing multiple copies of the default pointer in the array of pointers reduces the number of route indexes that can be stored in a fixed size memory.

[0016]FIG. 2A illustrates a worst case encoded subtree which requires twice the number of entries in the array of pointers than actual routes. The routes r1-r8 are assigned to leaf nodes 215, 217, 219, 221, 223, 225, 227 and 229 as follows: r1 to leaf node 215; r2 to leaf node 217, r3 to leaf node 219; r4 to leaf node 221; r5 to leaf node 223; r6 to leaf node 225; r7 to leaf node 227 and r8 to leaf node 229. The default route r0 is assigned to all of the other leaf nodes 216, 218, 220, 222, 224, 226, 228 and 230.

[0017]FIG. 2B illustrates a prior art bit map 30 and associated pointers stored in an array of pointers 50 representative of the subtree shown in FIG. 2A. As described in conjunction with FIG. 1A, the bit map 30 includes one bit 242 ¹-242 ¹⁶ for each leaf node in the subtree. The array of pointers 50 stores a pointer for each bit set to ‘1’ in the bit map 30. A bit is set ‘1’ to indicate that the route corresponding to the leaf node differs from the route stored for the previous leaf node in the map table 32.

[0018] Sixteen entries are used to store the eight pointers to routes r1-r8 and the default route r0. The pointer to the default route r0 is stored in eight of the sixteen entries 252 ¹⁻¹⁶.

[0019] A route corresponding to a node in the subtree is found by counting the number of ‘1’s in the bit map and incrementing the pointer by the total number of ones. For example, the entry corresponding to node 224 (FIG. 2A) is stored in entry 252 ¹². Storing the pointer to the default route in eight different entries decreases the available memory for storing pointers to routes.

SUMMARY OF THE INVENTION

[0020] A longest prefix match lookup table defining nodes of a tree searched for a route pointer corresponding to a prefix match is presented. The lookup table stores a binary tree representation of a key in a plurality of subtree levels. A portion of the bits of the key are searched in each subtree level.

[0021] Each subtree level includes a subtree memory and a mapper memory. The result of a search of each subtree level indicates whether a search must continue in a subtree in the next subtree level. The mapper memory stores pointers for nodes in a subtree. The subtree memory stores a subtree descriptor indexed by a subtree select from the previous subtree level. Instead of including a bit per node in the bottom level of the subtree, the subtree descriptor includes a bit for each node in the subtree. The bit corresponding to the node indicates whether a pointer for the node is stored in the mapper memory. By increasing the number of bits in the subtree descriptor, a pointer to a default route for the subtree shared by a plurality of nodes in the subtree is stored in a single entry in the mapper memory. Thus, the number of available locations for storing routes in the mapper memory is increased.

[0022] If the bit corresponding to a node in the subtree descriptor is set to ‘1’, a pointer is stored for the node in mapper memory. The subtree descriptor includes a level descriptor for each level in the subtree. Each level descriptor includes a bit for each node in the level. Each subtree level also includes mapper address logic. The mapper address logic performs a parallel search in each level descriptor for a node matching a search key and computes an offset to the pointer corresponding to the matching node.

[0023] The subtree descriptor may also include a block pointer. The block pointer may store an index to the first pointer in the mapper memory for the subtree. The combination of the offset and the block pointer provides the index to the pointer corresponding to the matching node.

[0024] The default route is stored for a root node of the subtree. The default route stored may be an indication to use another default route for a parent subtree.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0026]FIG. 1A illustrates a prior art bit map for representing the first level of a binary tree.

[0027]FIG. 1B illustrates a prior art lookup table implemented in cache memory;

[0028]FIG. 2A illustrates a worst case encoded subtree which requires twice the number of map table entries than actual routes;

[0029]FIG. 2B illustrates a prior art bit map representation of the subtree shown in FIG. 2A and associated pointers stored in the map table;

[0030]FIG. 3A is a four level subtree;

[0031]FIG. 3B illustrates a bit map representation of the subtree shown in FIG. 3A according to the principles of the present invention;

[0032]FIG. 3C is a block diagram of a subtree descriptor and associated routes for the subtree shown in FIG. 2A according to the principles of the present invention;

[0033]FIG. 4 is a block diagram of a bit map representation of the subtree shown in FIG. 2A stored in the subtree data shown in FIG. 3C;

[0034]FIG. 5 illustrates a subtree mapper storing a mapper entry corresponding to a leaf node in a subtree and a subtree memory storing a subtree descriptor for the subtree;

[0035]FIG. 6 illustrates an embodiment of the mapper address logic for computing the mapper index to a mapper entry corresponding to a leaf node in the subtree;

[0036]FIG. 7 is a block diagram illustrating an embodiment of the level N offset count logic shown in FIG. 6;

[0037]FIG. 8 is a circuit diagram of the embodiment of the mapper address logic described in conjunction with FIG. 6 and FIG. 7 for a subtree with seven levels; and

[0038]FIG. 9 is a flowchart illustrating a method for computing the mapper index implemented in the mapper address logic shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

[0039] A description of preferred embodiments of the invention follows.

[0040] U.S. patent application Ser. No. 09/733,627 filed on Dec. 8, 2000 describes a method and apparatus for storing a route for an Internet Protocol (“IP”) address in a multi-level lookup table, the contents of which are incorporated herein by reference. A multi-level search is performed to find a route index stored in a mapper in the lookup table which indexes a range of IP addresses corresponding to a range of leaves of a subtree.

[0041]FIG. 3A is a four level subtree. The four level subtree has 15 (2⁴⁺¹−1) nodes with 3 route nodes r1, r2, r3 and 2 subtree entry nodes s0, s1. One node labeled ‘a’ is in level 1, two nodes labeled ‘b’ and ‘c’ are in level 2. Four nodes labeled ‘d’, ‘e’, ‘f’, ‘g’ are in level 3. Eight nodes labeled ‘h’, ‘i’, ‘j’, ‘k’, ‘l’, ‘m’, ‘n’, ‘o’, ‘p’ are in level 4.

[0042]FIG. 3B illustrates a bit map representation of the subtree shown in FIG. 3A according to the principles of the present invention. Each node in the subtree shown in FIG. 3A is assigned a label. Each node has a corresponding bit position in the bit map. For each route node and subtree entry node in the subtree, the corresponding bits in the bit map are set to ‘1’. Thus, bit 350 corresponding to node a, the root node of the subtree is set to ‘1’, bit 352 corresponding to route r1 (node c) is ‘1’, bit 354 corresponding to route r3 (node d) is ‘1’, bit 356 corresponding to subtree entry s0 is ‘1’ and bit 358 corresponding to subtree entry s1 is ‘1’.

[0043] Thus, the bit map includes a bit for each node in the subtree instead of only leaf nodes as in the prior art coding scheme described in conjunction with FIGS. 2A and 2B.

[0044] A bit map for a subtree of h=3 has 15 (2⁴⁺¹−1) instead of 8 bits 2³ for a bit map which has a bit per leaf node. FIG. 3C is a block diagram of a subtree descriptor 300 and associated routes for the subtree shown in FIG. 2A. The subtree descriptor 300 includes subtree data 308 and subtree pointers 310. The subtree data 308 includes a bit for each node in the subtree. The subtree pointers 310 include a block pointer 320 for storing a pointer to a block of mapper entries in a memory allocated for storing routes for the subtree. By including a bit for each node in the subtree data 308 instead of just leaf nodes, a pointer to a default route, shared by a plurality of nodes in the subtree, is stored in one location for all nodes in the subtree. The pointer to the default route is stored in a mapper entry corresponding to the root of the subtree. The small increase in the size of the subtree data 308 due to the longer bit map decreases the number of mapper entries for storing the default route to one per subtree. Thus, the number of routes that can be stored is increased. By eliminating the duplicate storage of pointers to the default route, the same number of routes stored in the prior art map table described in conjunction with FIG. 2B can be stored in a smaller memory.

[0045] The subtree data 308 includes a bit for each node in the subtree. A bit for each node in the bottom level of the N-level subtree; that is, the leaf nodes is provided in level N nodes bitmap 312. A bit for each node in the level above the bottom level (N-1) is provided in level N-1 nodes bitmap 314 and a bit for the root of the subtree is provided in the level 1 node bitmap 316.

[0046] The subtree pointers field 310 includes a default index 318 for storing the default route for the subtree and a block pointer 320 for storing a pointer to the start of a block of mapper entries allocated for storing routes for the subtree. The default index 318 is described in co-pending U.S. application Ser. No. 10/004,280 filed on Oct. 31, 2001 entitled “Default Route Coding,” by David A. Brown, the contents of which are incorporated herein by reference.

[0047]FIG. 4 is a block diagram of a bit map representation of the subtree shown in FIG. 2A stored in the subtree data 308 shown in FIG. 3C. The subtree data 308 in the subtree descriptor is expanded to include a bit 404 for each node 200-230 in the subtree shown in FIG. 2A instead of only the leaf nodes as shown in the prior art coding scheme (FIG. 2B). The number of bits in the subtree data 308 is 2^(h+1)−1 where h is the height of the subtree. For example, a subtree of height 8 requires 511 bits (2⁹−1) in the subtree data to represent the subtree. The subtree data 308 is described in conjunction with FIG. 2A.

[0048] As shown in FIG. 2A, the height (h) of the subtree is 4. Thus, the number of nodes is 31 (2^(h+1)−1 where h=4). Bits 404 ¹-404 ¹⁶ correspond to leaf nodes 215-230 (FIG. 2A) in the bottom level of the subtree, and to the level N (where N=h+1=5) nodes bit map 312 (FIG. 3C). Bits 404 ¹⁷-404 ²⁴ correspond to nodes 207-214 in the next level of the subtree, and to the level N-1 (where N-1=h=4) nodes bitmap 314 (FIG. 3C). Bits 404 ²⁵-404 ²⁸ correspond to nodes 203-206 in level 3 of the subtree, bits 404 ²⁹-404 ³⁰ correspond to nodes 201, 202 in level 2 of the subtree and bit 404 ³¹ corresponds to node 200 at the root of the tree (the default route for the subtree) and the level 1 node bitmap 316 (FIG. 3C).

[0049] As shown, a pointer to each route r1-r8 in the subtree shown in FIG. 2A is stored in a respective mapper entry 406 ¹-406 ⁸ in mapper memory 402. Mapper entry 406 ¹ stores a pointer to r1 for leaf node 215 in the subtree. Mapper entry 406 ² stores a pointer to route r2 for leaf node 217 in the subtree. Mapper entry 406 ³ stores a pointer to route r3 for leaf node 219 represented by bit 404 ⁵ in the subtree data 308. Mapper entry 406 ⁴ stores a pointer to route r4 for leaf node 221 represented by bit 404 ⁷ in the subtree data 308. Mapper entry 406 ⁵ stores a pointer to route r5 for leaf node 223 represented by bit 404 ⁹in the subtree data 308. Mapper entries 406 ⁶, 406 ⁷ and 406 ⁸ store a pointer to routes for respective leaf nodes 225, 227 and 229. All of the other nodes in the subtree map to the default route r0. The default route r0 is stored once in mapper entry 406 ⁹ for the root of the subtree represented by bit 404 ³¹ in the subtree data 308 and for all nodes mapping to the default route.

[0050] Returning to FIG. 2A, the prior art subtree encoded using only leaf nodes requires 16 mapper entries 252. Continuing with FIG. 4, by storing the default route r0 for the subtree in only one mapper entry, the number of mapper entries used is reduced from 16 to 9 for the same four level subtree with 16 leaf nodes and eight routes shown in FIG. 2A. Thus, by not duplicating default routes stored in mapper memory, the available memory for storing routes is increased. In the example shown, seven mapper entries which would have been used to store the default route in the prior art (FIG. 2B) can be used for storing routes. The increase in the number of bits in the subtree descriptor is offset by the corresponding reduction in the number of mapper entries 306 used per subtree, to store the same number of routes stored in the prior art lookup table.

[0051] In one embodiment, the default route is stored in the default index 318 to facilitate updating the default route and allow the default route to be inherited from a parent subtree. An inherit indicator is stored in the default index 318 instead of the default route, to indicate that the default route corresponding to the root of the subtree is inherited from a parent subtree. A use default indicator is stored in mapper entry 406 ⁹ instead of the default route itself. Upon detecting the use default indicator stored in the mapper entry, the pointer to the default route stored in the default index or the inherited default route pointer is returned as the result of the search of the lookup table.

[0052]FIG. 5 illustrates a subtree mapper 502 storing a mapper entry 412 for a node in a subtree and a subtree memory 500 storing a subtree descriptor 300 for the subtree. A subtree index 508 forwarded from a mapper entry of a previous level selects the subtree descriptor 300 stored in the subtree memory 500 for the subtree. Mapper address logic 504 selects the mapper index 516 for the node dependent on the selected subtree descriptor 300 and the node index 510.

[0053] The node index 510 for a subtree of height N has N bits. for example, the node index 510 for a subtree of height 3 has 3 bits. The three bits identify the leaf node (i.e., the node at the bottom of the subtree) and all parent nodes up to the root of the subtree.

[0054] Returning to FIG. 3A a node index=‘000’ identifies mapper entries for node ‘h’ and parent node ‘d’ and ‘b’ and root node ‘a’. A search for a longest match begins with the leaf node ‘h’ identified by the node index 510.

[0055] The subtree descriptor 300 includes subtree data 308 (FIG. 3C) and subtree pointers 310 (FIG. 3C). The subtree data field 308 has one bit for each node in the subtree as described in conjunction with FIG. 3C. The subtree descriptor 300 also includes a pointers field 310 to allow for the storage of pointers to mapper entries in the subtree mapper 502 to provide access to the 256 mapper entries 412 that can be stored for an eight level subtree.

[0056] The subtree data 308 stored in the subtree descriptor 300 is forwarded to the mapper address logic 504. The mapper address logic 504 also receives a node index 510. The mapper address logic 504 determines the mapper index 516 to the mapper entry 412 corresponding to the node in the subtree dependent on the node index 510, the subtree data 308 and the subtree pointers 310 in the subtree descriptor 300 for the subtree. The mapper index 516 selects the mapper entry 412 for the node in the subtree.

[0057]FIG. 6 illustrates an embodiment of the mapper address logic 504 for computing the mapper index 516 for a mapper entry 412 (FIG. 5) corresponding to a node index 510 in the subtree. The mapper address logic 504 includes offset count logic 600⁰-600 ^(n) for each level in the subtree, where n=h+1, h being the height of the subtree. The offset count logic 600 counts the number of mapper entries lower than the selected node based on the number of bits set to ‘1’ in the subtree data 308 (FIG. 3) in the subtree descriptor 300 (FIG. 3C).

[0058] The total number of mapper entries lower than the mapper entry for the selected node is computed by summing the offsets 602 ¹-602 ^(n) output by the offset count logic 600 ¹-600 ^(n) for each level. The mapper index 516 is computed in the adder 610 by adding the pointer offsets to the block pointer 320 (FIG. 3C) stored in the subtree descriptor 300.

[0059]FIG. 7 is a block diagram illustrating an embodiment of the level N offset count logic 600 ^(n) shown in FIG. 6. As described in conjunction with FIG. 6, the level N offset count logic 600 ^(n) computes the offset from the block pointer 320 (FIG. 3C) to the node selected by the node index 604. The offset is the number of mapper entries stored in mapper memory for the subtree between the mapper entry for the selected node and the block pointer 320 (FIG. 3C) for the subtree.

[0060] The level N offset logic 600 ^(n) includes a level N thermometer decoder 700, a bitmap comparator 702 and a level N offset adder 704. If there is a mapper entry stored for the level N node, the level N thermometer decoder 700 selects all nodes lower than the leaf node index 604. A match N signal 708 is set to ‘0’. The match N signal is coupled to the bitmap comparator 702 in the level N-1 offset count logic 600 ^(n-1). The match N signal 708 set to ‘0’ disables the output of offset count logic 600 of lower levels. If there is no mapper entry stored for the level N node, all of the bits in the match bit map 706 are forced high. The match N signal 708 is set to ‘1’ to enable the output of offset count logic 600 of lower levels.

[0061] The bitmap comparator 702 compares each bit in the match bit map 706 with the respective bit in the level N nodes bitmap 312 stored in the subtree descriptor 300. The result of the comparison is output as the offset bitmap 710. The offset bitmap 710 indicates all mapper entries 406 that are stored in mapper memory 402 up to the selected node for the subtree. The level N offset adder 704 counts the number of ‘1’s in the offset bit map 710. The total number of ‘1’s is the level N offset 602 ^(n) of the mapper entry for the node from the block pointer 320.

[0062]FIG. 8 is a circuit diagram of the embodiment of the mapper address logic 504 described in conjunction with FIG. 6 and FIG. 7 for a subtree with h=7 and a total of 255 (2⁸−1) nodes. The subtree has eight levels 0-7, and corresponds to 7-bits [6:0] subtree. Offset count logic 800 ⁷corresponds to level N offset count logic 600 ^(n)=7. The mapper address logic 504 includes level 7 offset count logic 800 ⁷, level 6 offset count logic 800 ⁶ and level 1 offset counter 800 ¹ and level 0 offset logic 800 ⁰. Level 7 offset count logic 800 ⁷ includes a level 7 thermometer decoder 802 ⁷, 2⁷ (128) bit map comparator 804 ⁷ and level 7 offset adder 806 ⁷ as described in conjunction with FIG. 7.

[0063] The level 7 thermometer decoder 802 ⁷ corresponds to the level N thermometer decoder for level N, where N=7 which was described in conjunction with FIG. 7. The 2⁷(128) bitmap comparator 804 ⁷ corresponds to the bitmap comparator for level N, where N=7, described in conjunction with FIG. 7. The level 7 offset adder corresponds to the level N offset adder where N=7 described in conjunction with FIG. 7.

[0064] Level 7 of the subtree has 128 (2⁷) nodes. The thermometer decoder 802 ⁷ includes a 1 of 128 decoder 808, a plurality of AND-gates, one for each of the 128 outputs of the 1 of 128 decoder 808, a NOR gate 812 and a plurality of OR gates, one for each output of the plurality of AND-gates. The bit map comparator 804 ⁷ includes a plurality of AND gates, one for each bit in the level 7 nodes bit map. The operation of the mapper address logic 504 is explained using a numerical example. The bit map (for the most significant 21 bits of the 128 bitmap) stored in subtree data 308 (FIG. 3C) in the subtree descriptor 300 (FIG. 3C) are shown in Table 1 below: TABLE 1 Bit Map Value (Bit 0: Bit 20) SM⁷ ₀-SM⁷ ₂₀ 110101011101010110111 1 of 128 decoder 000000000000000010000 (First set of AND-gate outputs (810⁰-810²⁰) 000000000000000010000 Match Bit Map 111111111111111110000 (OR-gate outputs (814⁰-814²⁰) ) Offset Bit Map 110101011101010110000 (Second set of AND gate outputs (816⁰-816²⁰) )

[0065] The node index [6:0] is ‘001000’ indicating a search for the route corresponding to the 17^(th) node in the seventh level of the subtree. The level 7 nodes bit map SM⁷ ₀-SM⁷ ₁₂₇ for the first 21 nodes in the seventh level of the subtree is shown in Table 1. The bit map SM⁷ ₀-SM⁷ ₁₂₇ is stored in the level N (N=7) nodes bit map 312 (FIG. 3C) in the subtree data 308 (FIG. 3) in the subtree descriptor 300 (FIG. 3C). The SM⁷ ₁₆ bit is ‘1’ indicating that a mapper entry is stored for the node in mapper memory. The bitmap output from the 1 of 128 decoder 808 based on the leaf node index [6:0] set to ‘001 0000’ has bit SM⁷ ₁₆ set to ‘1’ and all other bits set to ‘0’ as shown in Table 1. Thus, all bits output from the “1 of 128 decoder” 808 are set to ‘0’ except the S_(M0) ¹⁶ bit which is set to ‘1’ indicating the selected node.

[0066] Each output from the 1 of 128 decoder 808 is compared with a respective bit of the level 7 nodes bitmap SM⁷ ₀-SM⁷ ₁₂₇ to determine if there is a mapper entry for the 17^(th) node; that is, if there is a mapper entry for the 17^(th) node in the 7^(th) level of the subtree. As shown in Table 1, only bit 16 of the bit map output from the plurality of the AND gates 810 ⁰-810 ¹²⁷ is set to ‘1’ indicating that there is a mapper entry for the 17^(th) node. Each of the AND gate outputs is coupled to a respective input of NOR gate 812. If any of the inputs to NOR gate 812 is set to ‘1’, the output of NOR gate 812 is set to ‘0’, indicating that there is a mapper entry corresponding to a node in the 7^(th) level. If the output of NOR gate 812 is set to ‘0’, a further search for a match in the level 6 nodes or nodes in any other upper levels of the subtree is not necessary because a node in the 7^(th) level is selected according to the longest match requirement when there are multiple matches in the subtree.

[0067] As shown, there is a mapper entry for the 17^(th) node in level 7 of the subtree, thus the output of the NOR gate 812 is set to ‘0’. The output of NOR gate 812 (match N 708 (FIG. 7) is coupled to level 6 offset logic 800 ⁶ and also coupled to one input of each of the plurality of 2-input OR gates 814 ⁰-814 ¹²⁷.

[0068] The 2-input OR gates select all bits in the level 7 nodes bitmap SM⁷ ₀-SM⁷ ₁₂₇ that are lower than or equal to the selected node (17^(th) node) selected by the 1 of 128 decoder 808. The match bit map 706 (FIG. 7) output from the OR gates 814 ⁰-814 ¹²⁷ is shown in Table 1. Bits 0:16 are set to ‘1’ and all other bits are set to ‘0’.

[0069] The bitmap comparator 702 includes a plurality of 2-input AND-gates 816⁰-816 ¹²⁷, one for each bit in the match bit map 706. Each AND gate 816 ⁰-816 ¹²⁷ compares one bit of the match bit map 706 with a respective bit of the level 7 nodes bitmapS_(M0) ⁷-S_(M) ⁷ ₁₂₇ 312 (FIG. 7). The result of the comparison provides the number of mapper entries stored in mapper memory up to and including the 17^(th) node in level 7 of the subtree. The bits of node bit map SM⁷ ₀-SM⁷ ₁₂₇ are ANDed with respective bits of match bit map. The bitmap output (offset bit map) 710 (FIG. 7) of the AND gates is shown in Table 1. The 128 bit, 1 bit adder 806 ⁷ counts the number of ‘1’s in the offset bit map 710 output from the AND gates 816 ⁰-816¹²⁷. The total number of bits set to ‘1’ is 11. Thus, an offset of 11 is added to the block pointer 320 (FIG. 6) to provide the mapper index 516 (FIG. 6) to the mapper entry for the 17^(th) node in level 7 of the subtree in mapper memory.

[0070] The speed of the mapper address logic 504 can be improved using other components such as look ahead adders, parallel versus serial logic and adder pre-decoders to reduce propagation delay. These techniques are well-known to those skilled in the art.

[0071]FIG. 9 is a flowchart illustrating a method for computing the mapper index implemented in the mapper address logic 504 shown in FIG. 8. FIG. 9 is described in conjunction with FIG. 8.

[0072] At step 900, each decoder 808, 810 in each respective offset count logic (FIG. 8) generates a node map in parallel for the selected node. The bit in the node map corresponding to the selected node is set ‘1’, all other bits are set ‘0’. Processing continues with step 902.

[0073] At step 902, the node map output by the decoder is compared with the node bit map for the respective level. If any of the resulting bits are ‘1’, there is a mapper entry for the node and processing continues with step 904. If not, processing continues with step 910 to compute the number of mapper entries used by the level.

[0074] At step 904, all mapper entries lower than the selected node in the level are selected in parallel in each level offset count logic 802 by selecting all bits above the selected node and comparing with the node bit map for the respective level. Processing continues with step 906.

[0075] At step 906, each level adder 806 computes the offset based on the number of mapper entries up to the selected node in the level. Processing continues with step 908.

[0076] At step 908, adder 610 adds the total number of mapper entries from all levels stored in mapper memory for nodes up to the selected node to the block pointer to provide the mapper index 516 to the pointer for the selected node. Processing is complete.

[0077] At step 910, there is no mapper entry for the selected node. All the mapper entries for the level are added to provide the offset to the first mapper entry in the next level. Processing continues with step 908.

[0078] A lookup table including a subtree descriptor encoding a subtree using one bit per node requires less overall memory than the prior art subtree descriptor encoding a subtree using one bit per leaf node. The reduction in memory for an embodiment capable of storing 256K routes in mapper memory is shown below in Table 2. TABLE 2 Prior Art New Subtree memory 32 K × 256 bits = 32 K × 294 bits = 9.1875 M bits 8 M bits (practical 32 K × 304 bits = 9.5 M bits) Mapper memory 512 K × 21 bits = 256 K × 21 bits = 10.5 M bits 5.25 M bits (practical (practical size = size = 256 K × 512 K × 24 bits = 24 bits = 6 M bits) 12 M bits) Total memory 18.5 M bits 14.4375 M bits (practical size = (practical size = 20 M bits) 15.5 M bits)

[0079] The number of bits per entry in subtree memory is increased from 256 bits to 294 bits. The 294 bit entry includes 256 bits of subtree data, a 20-bit default index and an 18-bit block pointer field. As shown in Table 2, the total memory is reduced from 20 Mega bits to 15 Mega bits.

[0080] The invention has been described for an embodiment in which the mapper address logic is implemented using decoders, adders and combinational logic (AND, NOR, OR gates). However, in an alternate embodiment, the mapper address logic can be implemented using a Content Addressable Memory (CAM) as is well known to those skilled in the art.

[0081] While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. A longest prefix match lookup table defining nodes of a tree searched for a route pointer corresponding to a route prefix match, the table comprising: a mapper memory which stores pointers for nodes in a subtree; and a subtree descriptor stored in a subtree memory, the subtree descriptor including a bit for each node in the subtree, the bit corresponding to the node indicating whether a pointer for the node is stored in the mapper memory, a pointer to a default route for the subtree shared by a plurality of nodes in the subtree being stored in a single entry in the mapper memory.
 2. The longest prefix match lookup table as claimed in claim 1 wherein a pointer is stored for a node in mapper memory, if the bit corresponding to the node in the subtree descriptor is set to ‘1’.
 3. The longest prefix match lookup table as claimed in claim 1 wherein the subtree has a plurality of levels and the subtree descriptor includes a level descriptor for each level, each level descriptor including a bit for each node in the level.
 4. The longest prefix match lookup table as claimed in claim 3 further comprising: mapper address logic which performs a parallel search in each level descriptor for a node matching a search key and computes an offset to the pointer corresponding to the matching node.
 5. The longest prefix match lookup table as claimed in claim 4 wherein the subtree descriptor further comprises: a block pointer, the block pointer storing an index to the first pointer in the mapper memory for the subtree, the combination of the offset and the block pointer providing the index to the pointer for the matching node.
 6. The longest prefix match lookup table as claimed in claim 1 wherein the default route is stored for a root node of the subtree.
 7. The longest prefix match lookup table as claimed in claim 6 wherein the default route stored for the root node is an indication to use another default route for a parent subtree.
 8. A method for providing a longest prefix match lookup table comprising: storing pointers for nodes in a subtree in a mapper memory; and including a bit for each node in the subtree in a subtree descriptor stored in a subtree memory, the bit corresponding to the node indicating whether a pointer for the node is stored in the mapper memory, a pointer to a default route for the subtree shared by a plurality of nodes in the subtree is stored in a single entry in the mapper memory.
 9. The method as claimed in claim 8 wherein a pointer is stored for a node in mapper memory if the bit corresponding to the node in the subtree descriptor is set to ‘1’.
 10. The method as claimed in claim 8 wherein the subtree has a plurality of levels and the subtree descriptor includes a level descriptor for each level, each level descriptor including a bit for each node in the level.
 11. The method as claimed in claim 10 further comprising: performing a parallel search in each level descriptor for a node matching a search key; and computing an offset to the pointer corresponding to the matching node.
 12. The method as claimed in claim 11 further comprising: storing an index to the first pointer in the mapper memory for the subtree in a block pointer in the subtree descriptor, the combination of the offset and the block pointer providing the index to the pointer for the matching node.
 13. The method as claimed in claim 8 wherein the default route is stored for a root node of the subtree.
 14. A method for encoding a subtree comprising the steps of: allocating a subtree descriptor in memory, the subtree descriptor including a bit for each node in the subtree; storing a pointer to a default route for the root in a single entry in mapper memory, the single entry shared by all nodes mapping to the default route; setting the bit corresponding to a root of the subtree in the subtree descriptor indicating that the default route is stored in the mapper memory; and setting the bit in the subtree descriptor corresponding to a node in the subtree having an entry in mapper memory other than the default route.
 15. A method for obtaining a longest prefix match from a longest prefix match table comprising the steps of: searching a subtree descriptor for the longest prefix match, the subtree descriptor including a bit for each node in a subtree, each bit of the subtree descriptor set to ‘1’ having an entry in mapper memory, a single entry in mapper memory storing a default route for the root, the single entry shared by all nodes mapping to the default route; and upon detecting the node corresponding to the detected node, computing a pointer to the mapper entry corresponding to the detected node dependent on the number of bits set to ‘1’ in the subtree descriptor for nodes in the subtree below the node corresponding to the longest prefix match. 